Patients with sustained alveolar hypoventilation, such as patients with central alveolar hypoventilation syndrome (Ondine's curse), defective chemoreflexes, obesity-hypoventilation syndrome, kyphoscoliosis, and neuromuscular disease, but also the large group of patients with chronic airflow limitation, can often breathe adequately while awake but hypoventilate during sleep, particularly during rapid eye movement (REM) sleep. Therefore, these patients require ventilatory assistance during sleep. In addition, some may require oxygen therapy, particularly during sleep.
However, from the clinical perspective, it is difficult to determine a correct degree of ventilatory support to ensure adequate ventilation during all sleep stages, particularly REM sleep, while avoiding excessive ventilatory support in the awake state or in non-REM sleep. Excessive support can lead to over-ventilation with vocal cord closure and, ultimately, sleep disruption. Excessive support is also uncomfortable to the awake patient. Equally difficult is selecting the correct amount of supplemental oxygen therapy. Patients need more supplemental oxygen during periods of hypoventilation than during other periods, but excessive oxygen therapy can be deleterious or expensive.
A volume cycled ventilator set at a fixed respiratory rate and set to deliver a chosen amount of ventilation may largely solve the under-ventilation problem. However, it introduces three new problems.
Firstly, it is necessary to experiment with various tidal volume settings to find settings that achieve the desired level of blood gases. A rough estimate can be made from first principles, based on the patient's weight, height, age, sex, etc. However, differences in metabolic rate, in particular, the gas exchanging efficiency of the lungs, can introduce very large errors. In current practice, expert clinical experience is required to make such an assessment, and usually the chosen target ventilation needs to be tested overnight and iteratively refined.
Secondly, such ventilators when correctly set are uncomfortable for most patients because the patient can only breathe at exactly the rate and depth set by the machine.
Thirdly, the ventilator may not be set accurately. If the ventilator is set to give slightly too much ventilation, the subject will be over-ventilated, leading to airway closure and very high airway pressures. Alternatively, if the ventilator is set to give slightly too little ventilation, the subject will feel air hunger.
The usual clinical compromise solution is to use a bilevel ventilator set to administer a fixed higher airway pressure during inspiration and another fixed lower airway pressure during expiration. The device is typically set to trigger from the expiratory pressure to the inspiratory pressure on detection of patient inspiratory airflow and to trigger back to the expiratory pressure on cessation of patient inspiratory airflow. A backup rate is provided for the case where the patient makes no effort within a given period.
This solution is a compromise for several reasons. Firstly, it is difficult to select a degree of assistance that will adequately support the patient during, for example, REM sleep, without over-ventilating the patient during non-REM sleep or while the patient is awake. We have made measurements of the degree of support that makes typical patients feel most comfortable during the daytime and found it to be much less than the degree of support that provides adequate ventilation during sleep. In many patients, the degree of support required during sleep, when delivered to the patient in the awake state, actually feels worse than no support at all. Secondly, the square pressure waveform is uncomfortable and intrusive in patients with normal lung function. Thirdly, it is necessary to empirically set the device while the patient sleeps, and it may take several iterations to find an adequate compromise. This procedure requires highly experienced staff and is very expensive.
One method of providing more comfortable ventilatory support is proportional assist ventilation. A device using this method seeks to provide a more comfortable pressure waveform that necessarily avoids over-ventilation because the patient must supply some effort, which is then amplified by the device. Unfortunately, this method will not work in the case of patients with absent or severely impaired chemoreflexes in sleep. This problem exists in people with Ondine's curse or obesity hypoventilation syndrome, or in people in whom the coupling between effort and result reduces dramatically during sleep, for example, patients with neuromuscular disease, where accessory muscle activity is completely lost during sleep. This problem also occurs in a very wide range of patients during REM sleep when there is routinely abnormal chemoreflex control, even in normal subjects.
The broad class of servo ventilators partially address the problem of the patient requiring much less support while awake than asleep. The physician specifies a target minute ventilation, and the device supplies sufficient support to deliver the specified minute ventilation on average. While the patient is awake and making large spontaneous efforts, the device will provide zero support. However, it will provide support as required during sleep. Further refinements of the servo-ventilator including the features of a smooth pressure waveform template, resistive unloading, low source impedance so that the patient can breathe more than the target ventilation if desired, and a minimum degree of support chosen to be comfortable in the awake state are taught in commonly owned International Publication No. WO 98/12965.
The above approaches all require the specification of a target ventilation, and either a respiratory rate or a backup respiratory rate. In addition, in patients requiring added supplemental oxygen, it is necessary to also specify the amount of added oxygen. Finally, using the above approaches, if the device is incorrectly calibrated, it will deliver a different minute ventilation than the one chosen.